The present invention relates to a power supply circuit that is of particular use for providing ac current to inductive loads as used, for example, in induction heating or melting applications wherein the inductive load may be subject to a short circuit that can produce, depending upon the topology of the supply, a high level of fault voltage across, or fault current through, switching devices used in the supply.
A conventional series-resonant dc-to-ac inverter power supply circuit for an induction furnace, or other load having a substantially inductive impedance, includes a dc power source and an inverter having one or more pairs of series-connected switch circuits. Each switch circuit comprises a series combination of an inductive reactor and switching device, such as a silicon-controlled rectifier (SCR), with an antiparallel diode connected across the switching device. The inductive reactor limits the rate of change of current over time through its associated SCR when it turns-on (commutates) and, consequently, is referred to as a di/dt reactor. The inverter""s SCRs are connected to an inductive load, such as an induction coil, and are alternatively gated (triggered) to switch between the non-conducting and conducting states. With this arrangement, each SCR in a pair of SCRs alternately allows current to flow through the induction coil in opposing directions, thus establishing an ac current flow through the coil.
Parallel-resonant dc-to-ac inverter power supply circuits are also used with inductive loads. However, series-resonant dc-to-ac inverter power supplies are preferred because of their superior controllability. Series-resonant dc-to-ac inverter power supplies are susceptible to short circuits in the inductive load. For example, when the load is an induction coil used in an electric induction furnace for metal melting, it is not unusual for spilled molten metal, or scrap metal being loaded into the furnace, to come into contact with the coil and cause at least a partial short circuit between two or more of the coil turns. The resulting instantaneous over-voltage condition across a non-conducting switching device in the inverter at the time of the short circuit can degrade or destroy the device. A known solution to the problem is to trigger the non-conducting switching device into conduction to eliminate the over-voltage condition across the device. However, a disadvantage of this approach is that it causes extremely high current to flow through the switching device which, in turn, generates significant heat in the device over a very short period of time. The switching devices are, in effect, forced to withstand the extremely high current in order to avoid being subjected to the over-voltage condition. The result of subjecting the switching device to these high current levels is degradation of the device and premature failure.
A solution to this problem is disclosed in U.S. Pat. No. 6,038,157. This patent teaches over-voltage protection of switching devices by adding a protective inductor in series with the load induction coil as illustrated in attached FIG. 1. Circuit 110 includes a dc power source comprising a rectifier bridge circuit 120 (shown diagrammatically), series filter inductor 174 and parallel filter capacitor 172, and two solid state switching device 130, arranged in inverse parallel configuration. Each switching device has one terminal connected to an output bus of the de power source. Antiparallel diode 132 is connected across each switching device. A suitable, but non-limiting, switching device is a gate-controlled semiconductor device, such as an SCR. A di/dt reactor 140 is connected in series between the pair of switching devices as shown in FIG. 1. Protective coil 150 has a first terminal connected to the approximate electrical midpoint of di/dt reactor 140, and a second terminal connected to a first terminal of load induction coil 160. The second terminal of load induction coil 160 is connected to the common connection between two series-connected commutation or tank capacitors 170 which, in series combination, are connected across the output buses of the dc power source. In an induction metal melting application, load induction coil 160 is typically wound around the exterior of the heating crucible. Direct current supplied from the power source is positively and negatively switched through switching devices 130 to supply an ac current to load induction coil 160. The current flowing through coil 160 generates a magnetic field that inductively couples with a metal load in the crucible. The magnetic field induces an eddy current in the metal load that heats the metal. Since protective coil 150 continuously carries full load current, it generates significant power losses that decrease the overall efficiency of the power supply circuit. Furthermore the losses increase with the switching frequency of the supply. In the event of a short circuit in load induction coil 160, the voltage applied to switching devices 130 is reduced by a voltage dividing circuit that comprises protective coil 150 and di/dt reactor 140.
The present invention solves the problem of premature failures of the switching devices from exposure to over-voltage conditions resulting from short circuits in the load induction coil without penalizing circuit efficiency under normal operation, and without subjecting the switching devices to high level of currents to avoid the over-voltage.
In one aspect, the present invention is a fault tolerant power supply circuit for an inductive load that protects sensitive power switching devices from excessive over-voltage conditions by straddling a pair of switching devices in the leg of an inverter circuit with a protective capacitive element. The protective capacitive element suppresses an over-voltage that would otherwise be applied across the switching devices in the event that a short circuit occurs in the load circuit.
In another aspect, the present invention is a fault tolerant power supply circuit that comprises a protective circuit for preventing the voltage across a non-conducting switching device from exceeding the peak allowable voltage of the non-conducting switching device during a short circuit in an inductive load. The protective circuit comprises a series connected blocking diode and protective capacitor, and a discharge resistor. The series combination of the blocking diode and protective capacitor is connected across the series combination of a pair of switch circuits. Each switch circuit comprises a switching device connected anti-parallel to an antiparallel diode. The discharge resistor may be connected across the protective capacitor or between the common connection of the series connected blocking diode and the protective capacitor, and the positive dc bus. A protective circuit can be used for each pair of switch circuits in power supplies with multiple pairs of switch circuits, such as full-bridge inverters.
In another aspect, the present invention is a fault tolerant power supply circuit that comprises a protective circuit for preventing the voltage across a non-conducting switching device from exceeding the peak allowable voltage of the non-conducting switching device during a short circuit in an inductive load. The protective circuit also clamps voltage overshoots across a switching device when an antiparallel diode transitions to reverse bias. The protective circuit comprises a series connected blocking diode and protective capacitor, and a series connected discharge resistor and choke. The discharge resistor is connected between the common connection of the series connected blocking diode and the protective capacitor, and the positive dc bus via the choke. A protective circuit can be used for each pair of switch circuits in power supplies with multiple pairs of switch circuits, such as full-bridge inverters.
Other aspects of the invention are set forth in this specification and the appended claims.